1. Field of the Invention
This invention relates to a surge protection device for protecting electric and electronic circuits from abnormally high voltages and currents caused by, for example, lightning, switching surges or the like.
2. Description of the Prior Art
A wide range of devices referred to as surge protection devices have been devised. Even the number of such devices that fall in the category of two-terminal surge protectors is considerable. The better of these are not limited to the function of clamping the voltage across the device terminals at a fixed breakdown voltage at the time of breakdown caused by occurrence of a surge (i.e. do not function simply as constant-voltage diodes). Instead they further exhibit negative characteristics when the device current that begins to flow at the time of the device breakdown increases to above the breakover current value. As a result, the voltage across the terminals after breakdown is shifted to a clamp voltage that is lower than the breakdown voltage. It therefore becomes possible to absorb large currents.
Among the surge protection devices of this type, some utilize an avalanche or Zenner breakdown mechanism, while others use the punch-through breakdown mechanism. Insofar as a general comparison goes, those using the punch-through breakdown mechanism are more advantageous in a number of ways. By providing a broad range of design options, they enable the breakdown voltage to be selected freely irrespective of impurity concentration, resistivity, thickness and the like of the semiconductor substrate used. They also allow various electrical properties such as junction capacitance and resistivity to be designed independently. From some time ago, the inventors have therefore focused considerable effort on the improvement of the breakover type surge protection device utilizing the punch-through principle. The results of this work, which was conducted from many different angles, are described, for example, in U.S. Pat. No. 5,083,185, U.S. Ser. No. 799,200, U.S. Ser. No. 799,075, Japanese Patent Appln. Public Disclosure Sho 61-259501, Sho 62-65382, Sho 62-65383, Hei 4-320066, Hei 4-320067, Hei 4-196358, Hei 4-199577 and Japanese Patent Appln. No. Hei 3-238950. The present invention takes still another approach to the improvement of the device utilizing the punch-though principle. As background information facilitating an understanding of the invention, the basic structure and operation of the punch-through type surge protection device will be explained with reference to FIG. 8.
The device has a first semiconductor region 1 constituted by a semiconductor wafer or a semiconductor substrate. The first semiconductor region 1 can be of either n or p conductivity type. In the illustrated device it is of n type. In view of various conditions involved in the fabrication of the individual device regions of a punch-through type device, it is somewhat more advantageous to use an n type substrate 1. A second semiconductor region 2 and a third semiconductor region 3 are successively formed on one principal surface of the semiconductor region 1, ordinarily by double impurity diffusion or selective ion-implantation. Since the second semiconductor region 2 has to form a pn junction with the first semiconductor region 1, it is selected to be of p type. In the case of a punch-through type device, it is preferable for it to be of a somewhat low concentration p type (p.sup.- type). The third semiconductor region 3 is of opposite conductivity type from the second semiconductor region 2 and forms a second pn junction with the second semiconductor region 2. In the illustrated case, it is therefore of n conductivity type. Since as explained later, however, the third semiconductor region 3 constitutes one end of the main device current path after breakdown, it preferably has high conductivity and, therefore, is preferable a high concentration (n.sup.+) semiconductor region.
A fourth semiconductor region 4 is further formed on the other principal surface of the first semiconductor region 1 (the bottom surface in the drawing), so as to face the second semiconductor region 2. The fourth semiconductor region 4 is of the same conductivity type as the second semiconductor region 2 and forms a third pn junction with the first semiconductor region 1. In the illustrated case, a fifth semiconductor region 5 of opposite conductivity type from the fourth semiconductor region 4 (indicated by a chain line) forms a fourth pn junction with the fourth semiconductor region 4. The presence of the fifth semiconductor region 5 will, however, be ignored in the initial part of the explanation. If the fifth semiconductor region 5 is assumed not to be present, the fourth semiconductor region 4 does not have to be of p.sup.- type but can have an ordinary level of impurity concentration.
In the interest of brevity, the semiconductor regions 1, 2, 3, 4 (and 5) will be hereafter be referred to simply as "regions".
A first ohmic electrode E.sub.1 is provided in common ohmic contact with the second region 2 and third region 3, via an opening in an insulation film 6, while a second ohmic electrode E.sub.2 is provided in ohmic contact with the fourth region 4, via an opening in an insulation film 7. As shown schematically in FIG. 8, first and second device terminals T.sub.1 and T.sub.2 are lead out from the first and second ohmic electrodes E.sub.1 and E.sub.2, respectively. The device is connected with the circuit to be protected through these device terminals.
Structurally speaking, the surge protection device 10 shown in FIG. 8 is formed with the regions 1, 2, 3 and 4 stacked vertically in the thickness direction of the first region 1. Moreover, as will be clear from the explanation of the device's operation given later, the device current resulting from surge absorption flows in the thickness direction of the first region, between the third and fourth regions. The device can therefore be said to be of the vertical type. In contrast, although not illustrated, there are also prior art surge protection devices of the lateral type in which the fourth region 4 is situated on the same principal surface of the first region 1 as the second and third regions 2 and 3, side by side therewith. Since there is little difference in operating principle between the vertical and lateral devices, however, only the vertical structure will be explained here.
In the sectional configuration of FIG. 8, when a surge voltage arises across the first and second device terminals T.sub.1, T.sub.2 (first and second ohmic electrodes E.sub.1 and E.sub.2) at a relatively large magnitude and in such phase as to apply a reverse bias across the first pn junction between the first region 1 and the second region 2 (in the illustrated case, such that the first device terminal T.sub.1 side becomes negative), the upper extremity of the depletion layer produced at the pn junction between the first and second regions reaches the third region 3, whereby punch-through is established. This is because when the region 2 is of low concentration p.sup.-, type the depletion layer grows mainly toward the third region 3.
When punch-through occurs, minority carriers (from the viewpoint of the first region 1) are injected into the first region 1 from the fourth region 4 via the forward biased third pn junction. Since the injected minority carriers collect in the second region 2, device current begins to flow. The voltage at which this punch-through operation starts is designated as the breakdown voltage V.sub.BR on the voltage axis in the operating characteristics of these surge protection devices shown in FIG. 9.
Even if the second region 2 and the third region 3 should be shorted at their surfaces by mutual connection with the second device terminal T.sub.2, following the start of minority carrier flow in this manner, once the device current begins to flow via the second region 2 and rises to the point that the product between itself and the resistance along the path thereof in the second region 2 (the voltage drop) becomes equal to the forward voltage at the pn junction formed between the second region 2 and the third region 3, the pn junction turns on so that minority carriers (from the viewpoint of the second region 2) are injected from the third region 3 into the second region 2. This injection of minority carriers into the second region 2 causes the device current flowing between the first and second device terminals T.sub.1, T.sub.2 to become even larger, as indicated in FIG. 9 by the portion of the characteristic curve which rises rapidly in the direction of the current axis. Since this in turn promotes the injection of minority carriers from the fourth region 4 into the first region 1, a positive feedback is obtained. Thus, as can be seen from the characteristic curve in FIG. 9, when the current flowing between the first and second device terminals T.sub.1, T.sub.2 becomes greater than the value indicated as the breakover current I.sub.BO, the occurrence of positive feedback within the device manifests itself in the form of a negative resistance characteristic. As a result, the voltage across the first and second terminals T.sub.1, T.sub.2 shifts to a clamp voltage (or ON voltage) V.sub.P that is lower than the breakover voltage V.sub.BO at which breakover commenced and also lower than the breakdown voltage V.sub.BR at which punch-through first started. Therefore, the device is able to absorb large surge currents while holding down the amount of heat it generates.
The maximum current which the surge protection device can absorb across its first and second terminals T.sub.1, T.sub.2 is generally referred to as its "surge absorption capacity" I.sub.PP. On the other hand, the minimum device current capable of maintaining the device in its on state after it has once turned on is called its "hold current" I.sub.H. Stated differently, once the surge has subsided and a current equal to or larger than the hold current I.sub.H no longer flows through the device, the device automatically resets itself (turns off) to the state at the beginning of this explanation. Because of this, the hold current I.sub.H is also referred to as the "turn-off current."
As will be understood from the foregoing, the device can absorb surges of only one polarity. Specifically, it can absorb a surge only if it causes the first device terminal T.sub.1 to become negative. It can therefore be called a "unipolar" surge protection device. To realize a "bipolar" surge protection device able to absorb surges of both polarities (irrespective of which of the first and second device terminals T.sub.1 and T.sub.2 becomes negative), it suffices to constitute the fourth region 4 to be substantially identical with the second region 2 (and thus have a somewhat low p.sup.- type impurity concentration as shown in FIG. 8), form in the fourth region 4 a fifth region 5 (as indicated by the chain line) that is substantially identical with the third region 3, and short the surfaces of the fourth and fifth regions 4 and 5 with the second ohmic electrode E.sub.2. In this specification, the terms "unipolar" and "bipolar" are used with regard to the polarity of the surges that a device is able to absorb. In the following description, therefore, the term "unipolar surge protection device" will be used to indicate surge protection devices able to absorb surges of only a specific single polarity and the term "bipolar surge protection device" will be used to indicate surge protection devices able to absorb surges of either polarity. This convention will be followed without use of any other mark or sign for emphasis.
In the bipolar surge protection device provided with the fifth region 5, if the surge polarity is reversed from that applied to the just-explained unipolar surge protection device capable of absorbing surges of only a specified polarity, specifically if the surge polarity becomes such that the first device terminal T.sub.1 in the drawing becomes positive, the condition is substantially operationally equivalent to that in which the third region 3 is not present. Therefore, the fifth region 5 performs the function of the third region 3 explained above, the fourth region 4 performs that of the second region 2 described earlier, and the second region 2 performs that of the fourth region 4 described earlier. As a result, the voltage-vs-current characteristic curve becomes as shown in the third quadrant of FIG. 9 and, with respect to the origin, is symmetrical to the characteristic curve in the first quadrant. Insofar as the second and fourth regions 2 and 4 and the third and fifth regions 3 and 5 are fabricated to be identical as regards geometry, physical properties and electrical characteristics, the breakdown voltage -V.sub.BR, breakover voltage -V.sub.BO, breakover current -I.sub.BO and hold current -I.sub.H in the third quadrant will be the same in absolute value to those in the first quadrant.
The foregoing explanation applies not only to the illustrated vertical surge protection device but also, without substantial modification, to the lateral surge protection device in which the second region 2 and the fourth region 4 are provided at laterally separated positions on the same principal surface. In a still more practical configuration, in order to increase the surge absorption capacity I.sub.PP, for example, a plurality of third regions 3 are provided side by side in the second region 2, as shown in FIG. 10. Where the aforementioned bipolar surge protection device is to be constituted, a plurality of the fifth region 5 are also provided in the fourth region 4. (When FIG. 10 is viewed in terms of the symbols in parentheses, it represents the structure on the fourth region 4 side.)
The prior art surge protection devices fabricated in accordance with the aforesaid basic structure are relatively free of problems insofar as they are constituted to have small surface area. However, when the area is enlarged so as to enhance the surge absorption capacity I.sub.PP or when an attempt is made to achieve more uniform current and characteristics by providing a plurality of third regions (and also fifth regions in the case of a bipolar surge protection device) in the manner of FIG. 10, it is frequently found that the surge absorption capacity I.sub.PP turns out to be smaller than expected and that the clamp voltage V.sub.P is not reduced as far as expected. Moreover, since leak paths occur in many of the devices, the production yield is not particularly high. An investigation of the cause of these problems provided the following knowledge.
It was discovered that in the devices in which the above-mentioned problems arose geometric steps or irregularities as indicated by the phantom circles marked C in FIG. 10 occurred at the second and fourth regions 2 and 4 as a feature of their relationship with the third and fifth regions 3 and 5. In particular, when the third and fifth regions 3 and 5 were formed with respect to the second and fourth regions 2 and 4 by double impurity diffusion, the push-out effect of the later formed impurity regions 3 and 5 tended to cause the formation of the steps C in the second and fourth regions 2 and 4. Because of this, steps C frequently formed under the corner portions where the side and bottom surfaces of the second or fourth regions 2 and 4 meet. (For reasons of convenience, the figures show the bottom surfaces of the fifth regions 5 facing upward. Hereinafter in this specification "bottom surface" will be termed with respect to the third regions 3 to mean the surface thereof parallel to the principal surfaces of the first region 1 and in contact with the second region 2 and with respect to the fifth region 5 to mean the surface thereof parallel to the principal surfaces of the first region 1 and in contact with the fourth region 4.
In a surge protection device of the type using punch-through which is the topic of this discussion the breakdown voltage and other such ratings have been designed on the assumption that the thickness and impurity concentration of the regions in which punch-through occurs are constant throughout. Therefore, if the geometry of the second and fourth regions 2 and 4 should come to deviate from the specification values during some step in the fabrication process in the foregoing manner (particularly if non-uniformity should arise in the thickness of portions extending in the planar direction), there will naturally be lack of uniformity in the device current which flows at the time of surge absorption. This has made it impossible to achieve a large surge absorption capacity etc.
In actual practice, it has been found that unless an expensive, high-precision production system is configured, the same problems also occur for reasons other than the push-out effect. Particularly when the region in which punch-through is to occur is large, any unevenness arising at the time of impurity diffusion, unevenness in the pn junction depth and the like make it extremely difficult to achieve uniform punch-through throughout.
The present invention was accomplished in the light of the foregoing circumstances and has as its object to provide a surge protection device whose characteristics are caused to be extremely near design values by eliminating or reducing non-uniformity apt to occur in the second region (in the case of a bipolar surge protection device, in the second and fourth regions) in which a depletion layer grows at the time of punch-through associated with a prescribed breakdown operation.
For achieving this object, in a surge protection device having a basic sectional structure as shown in FIG. 8 or 10, the present invention configures the second region as a combination of two regions, namely a punch-through generation region portion in which growth of the depletion layer associated with the start of breakdown is positively promoted and a punch-through suppression region portion in which growth of the depletion layer is positively suppressed, disposes the punch-through suppression region portion to cover the corners of the third region, and disposes the punch-through generation region portion only between the flat facing portions of the third and first regions.
While the punch-through generation region portion and the punch-through suppression region portion are of the same conductivity type, they differ in that the punch-through generation region portion has a lower impurity concentration. In addition, the distinction between the two portions can be achieved by making the punch-through generation region portion thicker than the punch-through suppression region portion. Still further, the distinction can be achieved by imparting both the difference in impurity concentration and the difference in thickness.
The invention can further be applied to a surge protection device having a plurality of third regions. In this case, it suffices to provide each of the third regions with the aforesaid punch-through generation region portion and punch-through suppression region portion. Alternatively, it is possible to provide at least one third region with a punch-through generation region portion and to cover all of the other third regions with punch-through suppression region portions which cover the bottom surfaces and corners thereof (or enclose them).
The invention also defines the impurity concentration and size of the punch-through suppression region portion such that the increase in device current flowing between the first and second ohmic terminals after generation of punch-through in the punch-through generation region portion produces a voltage effect which is large enough to turn on the aforesaid pn junction.
The present invention can also be applied to improve the earlier described bipolar surge protection device, provided that configurational modifications are made in line with configuration of the bipolar device. More specifically, for improving a prior art surge protection device provided with a fourth region substantially the same as the second region and a fifth semiconductor region substantially the same as the third region so as to enable absorption of surges of either polarity, the second region is, as explained earlier, constituted as a combination of a punch-through generation region portion and a punch-through suppression region portion, and, in addition, the fourth region is also constituted as a combination of a punch-through generation region portion and a punch-through suppression region portion of higher impurity concentration in such manner that the punch-through suppression region portion covers the corners of the fifth region, the punch-through generation region portion being provided only between the flat facing portions of the fifth and first regions.
The various modifications relating to the punch-through generation region portion and the punch-through suppression region portion of the second region of the earlier described unipolar surge protection device can also be applied to the punch-through generation region portion and the punch-through suppression region portion of the fourth region of the bipolar surge protection device.